TEM study of sodium bismuth titanate and bismuth nanoparticle growth under the electron beam irradiation

The lead-containing ceramic, lead zirconate titanate (PZT), is widely used in actuators, sensors and transducers as a result of its excellent piezoelectric properties (Kuyama M 1985). The toxicity of lead and lead oxides, however, has serious harmful effects on human health and the environment. Therefore, it is critical to develop replacement lead-free ceramics with comparable piezoelectric properties to those of PZT (Dorcet V et al 2008). Sodium bismuth titanate (NBT), is one of the promising candidates. This project investigates the atomic structure of NBT and the related compounds, NBT doped with barium titanate (NBT-BT), with the aim to understand their properties. Here we use a variety of transmission electron microscopy techniques to examine the microstructure of NBT and NBT-BT at room temperature, and in particular to determine the nanoscale deviations from the average structures. We found that the evidence for local structure modulation could be observed both in their high resolution transmission electron microscope (HRTEM) images and the corresponding diffraction patterns. HRTEM images suggest that the local structure consists of small "platelets" with an average length of 5 nm and an average width of 1 to 2 atomic planes. These platelets are consistent in their geometry with the diffuse streaks in the corresponding diffraction patterns, suggesting that they may be the cause of the diffuse streaks. Meanwhile, the diffuse streaks in the diffraction patterns were found to be present, even after the early stage of beam damage. During the course of our research, bismuth nanoparticles (BiNPs) were observed forming under electron beam irradiation in a transmission electron microscope (TEM) in both pure NBT and NBT-BT samples. Since this important effect has not been reported before for these materials, a detailed study was carried out on one kind of doped piezoelectric ceramics -sodium bismuth titanate doped with barium titanate (NBT-BT) at room temperature to characterize the microstructure of the BiNPs formed under electron irradiation. The study also investigated the dynamics of the structure modification during the growth process. It was found that the growth of bismuth nanoparticles could be seen within a few minutes of the exposure of the NBT-BT sample to the electron beam. In addition, the growing BiNPs were observed to rotate under the beam (rather than undergoing a reconstruction of their structure). After the BiNPs grew to a size in diameter of around 20 nm, the rotating effect ceased and the bismuth nanoparticles were observed seated steadily on the matrix.